1. Field of the Invention
The present invention relates to organic electroluminescent (EL) devices, and specifically to a full-color organic EL panel including at least red, green, and blue organic EL devices.
2. Description of the Related Art
Displays using organic EL devices have recently been intensively studied. Organic EL devices have advantages such as high luminance, wide viewing angles, and low power consumption, and have attracted attention as an alternative to liquid crystal displays for mobile applications.
Organic EL devices are separately driven for displaying. In general, organic EL devices emit light when supplied with current from a power supply disposed outside a display region where the devices are arranged. The current flows to a ground potential across electrodes of the organic EL devices, which are connected between the power supply potential and the ground potential, so that the devices can emit light.
Organic EL devices operate by passive-matrix driving, in which the intersections of orthogonal stripe electrodes are sequentially driven to emit light, or by active-matrix driving, in which thin-film transistors (TFTs) assigned to the individual devices control the driving thereof to emit light.
Japanese Patent Laid-Open No. 8-54836 discusses an example of an active-matrix light-emitting device. This light-emitting device includes an electron-injecting electrode connected to a drain electrode of a TFT via a contact hole, a hole-injecting electrode disposed opposite the electron-injecting electrode, and an organic thin-film layer disposed between the opposing electrodes. The hole-injecting electrode is formed of indium tin oxide (ITO), which is a typical transparent conductive material, so that light can be emitted through the hole-injecting electrode. The organic thin-film layer is supplied with a current controlled by the TFT to emit light.
FIG. 5 is a schematic diagram of an example of a conventional circuit for an active-matrix display. This circuit includes organic EL devices 7, a data-line drive circuit 8, a scanning-line drive circuit 9, data lines 10, scanning lines 11, switching TFTs M1 for controlling light emission of the devices 7, drive TFTs M2 for controlling the light output power of the devices 7, and hold capacitors C1 for maintaining the light emission.
The organic EL devices 7, the switching TFTs M1, the drive TFTs M2, and the hold capacitors C1 constitute individual pixel circuits which are arranged in a matrix. The organic EL devices 7 each have an anode A connected to a power supply potential VCC via the drive TFT M2 and a cathode K connected to a ground potential CGND.
Full-color images can be outputted by placing a color filter of the three primary colors of light, namely, red (R), green (G), and blue (B), on organic EL devices that emit white light, or by arranging organic EL devices that emit red, green, or blue light alternately in a dot matrix. The use of color organic EL devices is more advantageous in providing a display with higher luminance because they require no color filter and thus cause no loss of light. The optimum thickness and material of the color organic EL devices are different for each of the three primary colors. According to U.S. Pat. No. 6,864,628, red, green, or blue light-emitting devices include a plurality of hole-transporting layers to provide the optimum configuration for each color.
Battery life time is important for organic EL displays for mobile applications. Therefore, increased luminous efficiency and decreased drive voltage are required of organic EL devices used for the displays to reduce power consumption. Also, an extended device life time is required for long-term use, and cost reduction is required for competition with liquid crystal displays.